= Gravitational instability
Gravitational instability is the growth of a perturbation because its increased self-gravity overcomes the opposing support or restoring response. For a uniform nonexpanding gas, linearization gives the <Jeans instability> dispersion relation $\omega^2=c_s^2k^2-4\pi G\rho$: sufficiently long wavelengths grow rather than oscillate. In an expanding universe the background and <Hubble friction> modify the growth rate; a pressureless growing density mode can eventually become nonlinear. Rotation, magnetic stress, pressure and geometry can all alter the stability threshold in other systems.
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